Q3865 Trend History Rollover Time

My trend history files are set to roll over
at a specific time, but they do not seem to roll over at that time.
The history files' time stamp shown in the Windows Explorer is one
or more hours off, and the time varies from trend tag to trend tag,
even though they have the same time and period set in the Trend Tag
definition.

Solution:

There are two reasons for this:

1. When you open the Trend Tags form in the Citect Project
Editor and press F2 for extended options, you can set the trend
time. This time is in the Greenwich Mean Time (GMT) time zone, not
your local time zone. For example, Eastern USA standard time (EST)
is GMT - 5 hours. During daylight savings time it is GMT - 4 hours.
So, a trend set to roll over at 00:00:00 (midnight) will actually
roll over at 7:00 PM (8:00 PM daylight savings time). This affects
the beginning and end time of the data recorded in each history
file.

2. The Citect trend server caches trend sample data to improve
performance by reducing the amount of disk activity. The cache
size, trend sample rate, and number of trend tags determine how
long values can be cached before being written to disk. In Citect
5.31 and later there are 9 separate caches, each handling a
different range of sample rates. The size of each cache is set by
the [Trend]CacheSize<x> parameter (where x is 0 to 8). In
Citect 5.30 and older there is only one cache. Its size is set by
the [Trend]TrendBufSize parameter. Look up the parameter(s)
appropriate to your Citect version in the Citect help for more
information. The cache size affects the Windows created/modified
timestamps on the trend data files, but does not affect the
beginning and end times of the samples recorded in each file.